U.S. patent number 4,050,954 [Application Number 05/670,508] was granted by the patent office on 1977-09-27 for surface treatment of semiconductor substrates.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Jagtar S. Basi.
United States Patent |
4,050,954 |
Basi |
September 27, 1977 |
Surface treatment of semiconductor substrates
Abstract
The invention comprehends establishing a hydrophilic surface on
polished semiconductor wafers, such as silicon, after polishing
(e.g. silica polishing) by oxidation and hydrolysis of the wafer
surface for conditioning thereof for post-polishing cleaning.
Inventors: |
Basi; Jagtar S. (Fishkill,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
24690679 |
Appl.
No.: |
05/670,508 |
Filed: |
March 25, 1976 |
Current U.S.
Class: |
134/2; 134/28;
257/E21.224; 257/E21.228 |
Current CPC
Class: |
H01L
21/02043 (20130101); H01L 21/02052 (20130101) |
Current International
Class: |
H01L
21/306 (20060101); H01L 21/02 (20060101); C03C
023/00 () |
Field of
Search: |
;134/2,3,28 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
J Electrochem. Soc., Solid-State Science & Technology, vol.
120, No. 9, Sept. 1973 - "Silicon Surface Contamination: Polishing
and Cleaning", R. L. Meek et al., pp. 1241-1246. .
Solid State Technology, Nov. 1975, "LSI Wafer Cleaning Techniques",
D. Tolliver, pp. 33-36. .
Russian - Journal of Applied Chemistry, 31-11833 (1958) pp.
1183-1190, "Influence of the pH of the Medium and the Temperature
on the Proportions of Active Oxidizing Agents in Hypochlorite
Solutions in Chlorination and Bleaching Processes" by I. E.
Flis..
|
Primary Examiner: Kellogg; Arthur D.
Attorney, Agent or Firm: Powers; Henry
Claims
What is claimed is:
1. In a method of cleaning a silicon wafer polished with a metal
oxide polishing medium, the improvement comprising: establishing
hydrophilic surfaces on said wafer by oxidation thereof with an
oxidizing agent selected from the group consisting of alkali metal
hypohalites, and dichromates and permanganate salts of alkali
metals in sulfuric acid, and including the step of treating the
hydrophilic formed surfaces with an NH.sub.4 OH solution.
2. The method of claim 1 wherein said metal oxide is silica.
3. The method of claim 2 wherein said oxidizing agent is sodium
hypochlorite.
4. The method of claim 1 including the step of converting said
hydrophilic surfaces to a hydrophobic state by treatment with
hydrofluoric acid solution.
5. The method of claim 4 including the further step of
re-establishing a hydrophilic condition to said surfaces by
retreatment with said oxidizing agents.
6. The method of claim 5 wherein said metal oxide is silica.
7. The method of claim 6 wherein said oxidizing agent is sodium
hypochlorite.
8. In a method of cleaning silicon wafers polished with a metal
oxide polishing medium, the improvement comprising:
a. establishing hydrophilic surfaces on said wafers by oxidation
with an oxidizing agent selected from the group consisting of
alkali metal hypohalites, and dichromate and permanganate salts of
alkali metals in sulfuric acid;
b. treating said hydrophilic surfaces with an NH.sub.4 OH solution
to remove metal ions therefrom remaining from said polishing
medium,
c. establishing hydrophobic surfaces on said wafers by treatment
with hydrofluoric acid solution and concurrently removing oxides
therewith from said surfaces; and
d. reconverting said surfaces to a hydrophilic condition with
re-treatment with said oxidizing agent.
9. The method of claim 8 wherein said metal oxide is silicon
dioxide.
10. The method of claim 9 wherein said oxidizing agent is sodium
hypohalite.
11. The method of claim 8 wherein said improvement is repeated at
least once.
12. The method of claim 11 wherein said metal oxide is silicon
dioxide.
13. The method of claim 12 wherein said oxidizing agent is sodium
hypohalite.
Description
FIELD OF THE INVENTION
This invention relates to the conditioning of semiconductor wafers,
and more particularly to treatment of semiconductor surfaces for
cleaning thereof after polishing.
DESCRIPTION OF THE PRIOR ART
Semiconductor devices such as monolithic circuits, diodes, passive
devices and the like, are formed by various additive techniques,
such as diffusion and epitaxial growth, in the planar surfaces of
semiconductor substrates. For such semiconductor fabrication,
polished silicon wafers, free of undesirable crystal defects and
surface imperfections, is a basic requirement. Polishing and
cleaning of the wafers is normally the last process in preparation
of the wafers for device fabrication.
The nature of the wafer surface after polishing is determined
mainly by the particular polishing process employed, e.g. the
nature of the polishing slurry, polishing temperature, pressures
relative to polishing pads, wafer holding techniques during
polishing, etc. Normally, the slurry and polishing reaction
products are deposited and retained on the polished wafer surface
to varying degrees on completion of the polishing step. As a
result, the polished wafers require extensive cleaning before
further use for device fabrication.
For metal oxide-based (e.g. silica) polishing process, the cleaning
operations heretofore have employed, rinsing and water wheeling
wafers with water and surfactants, demounting and dewaxing wafers
with various organic solvents, hot acid treatments with nitric
acid, sulfuric acids and the like, dilute hydrofluoric acid
treatments, water rinsing, brush cleaning and drying. Although such
cleaning operations have heretofore been adequate, they are
characterized with shortcoming with increasing requirement for
densification of integrated circuits.
SUMMARY OF THE INVENTION
It has been found that inadequacies in cleaning of silicon wafers
polished with metal oxide (e.g. silica, SiO.sub.2 based slurries
are due to the induced hydrophobic surface on and/or polishing
particles embedded in the silicon wafer. Also, colloidal or
microcrystalline polishing slurry particles tend to stay on the
wafer surface in an extremely thin layer which, if allowed to dry,
forms a thin film which heretofore has resisted complete removal by
conventional cleaning operations.
Analysis has indicated that the hydrophobic nature of metal oxide
(e.g. silica) polished wafers is probably due to the formation of
--Si--O--Si-- bonds on the surface as a result of the relatively
high temperatures produced by the mechanically abrasive interaction
of the slurry with the silicon surface, and also reaction with the
base where employed in conjunction with metal oxide polishing
composition. Fast flushing and/or scrubbing helps to remove the
slurry film to a significant extent to produce a wafer that
normally appears to be clean under normal light conditions.
However, close observation under bright light, or other refined
techniques, shows large amounts of residue on the wafer surface
which is resistant to scrubbing operations which if too vigorously
applied can cause sub-surface damage in the silicon wafer. The
extent of this damage is basically dependent on the particular
equipment, processing conditions and solvents employed.
It has been found that contamination can be significantly removed
from polished silicon wafer surfaces by controllably converting the
wafer surfaces to hydrophilic (and optionally to hydrophobic again)
conditions during cleaning operations.
Specifically, removal of polishing slurry particles and heavy metal
ions can be effected by establishing a hydrophilic surface on the
wafer. Analysis indicates that this is accomplished by alteration
of the nature of the bonding of the oxide, which appears to be as
follows: ##STR1## Such conversion of the wafer surface to a
hydrophilic state can be obtained by the use of oxidizing agents,
alkali metal hypohalites and the dichromate and permanganate in
sulfuric acid. These oxidizing agents not only convert the silicon
surface to hydrophilic form but can also coagulate colloidal silica
(when used as the polishing medium) which can then be removed by
mere water rinsing.
This coagulation of silica with the hypohalite oxidizing agents is
clearly contrasted to the coagulation with alkali metal halides
(e.g. NaCl, NaBr, etc.) which coagulate silica in a form which
precipitates on the silicon surface in thin films which could not
be removed by simple rinsing.
Also, as is known, electrochemical equillibrium calculations of
Si/H.sub.2 O systems show that silicon is an extremely strong
reducing agent, whereby in the presence of strong oxidizing agents,
such as sodium hypochlorite, the silicon surface is also oxidized.
As a result, this facilitates the removal of any silica particles
which become embedded in the silicon surface during polishing
operations. Strong oxidizing agents also oxidize organic
contaminants.
The oxidizing operation is then followed by treating the silicon
surface, by immersion, in dilute NH.sub.4 OH, water rinse and spin
drying. The use of the ammonium hydroxide enables the removal by
complex ion formation with any heavy metal ions remaining on the
silicon surface as a result of the polishing operation or the
previous cleaning step employing oxidation. Also, the ammonium
hydroxide facilitates the removal of sodium ions. Any ammonia
adsorbed on the silicon surface readily volatilizes without leaving
any harmful residues. Also, the anti-static nature of ammonia
solution helps to remove adhering particles from the wafer
surface.
Accordingly, it is an object of this invention to provide a novel
method of producing semiconductor surfaces of uniform high
quality.
It is another object of this invention to provide a novel method of
conditioning semiconductor surfaces for device fabrication.
It is also an object of this invention to provide a novel method of
removing contamination from polished surfaces of
semiconductors.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
The following processing schedules show various steps involved in
practicing this invention for cleaning silicon wafers polished with
Monsanto Company's trademarked silica sol composition "Syton HT-50"
in a free polishing machine such as described in Goetz et al., U.S.
pat. No. 3,691,694 including modifications thereof in accordance
with the planetary configuration of Goetz et al. shown in the IBM
Technical Disclosure Bulletin (e.g. TDB), pp. 1760-1761, vol. 15,
No. 6, November 1972.
EXAMPLE 1
A. immerse for 3 minutes in 1% aq. NaClO solution.
B. rinse for 2 minutes in flowing DI (deionized) water.
C. immerse for 30 seconds in aq. 3% NH.sub.4 OH solution.
D. rinse for 2 minutes in flowing DI water.
E. immerse for 15 seconds in aq. 5% HF to establish a hydrophobic
condition on the wafer surface and remove silicon oxide
therefrom.
F. rinse for 2-3 minutes in flowing DI water.
G. immerse for 3 miutes in aq. 1% NaClO.
H. repeat Step B.
I. repeat Step C.
J. rinse for 5 minutes in flowing DI water.
K. spin dry.
In the above process, wafers with excellently cleaned surfaces were
obtained after Step D which were acceptable for device fabrication.
However, examination indicated the presence of some particles on
the wafer surface which were hard to remove by brush cleaning,
without any significant difference in particle removal even though
the immersion time in NaClO of Step A was increased. Removal of
these particles is obtained on completion on immersion in HF in
Step. E.
EXAMPLE 2
Fifteen free polished silicon wafers (as in Example 1) were cleaned
in accordance with the following schedule.
A. immersion for 3 minutes in aq. 5% NaClO.
B. rinsing for 2 minutes in flowing DI water.
C. immersion for 30 seconds in aq. 3% NH.sub.4 OH solution.
D. rinsing for 5 minutes in flowing DI water, and spin drying.
E. brush cleaning.
Of the fifteen wafers cleaned, four were rejected for scratches and
edge chips, not due to the cleaning operation, but resulting from
polishing operations.
The remaining eleven wafers easily met the high criterias set for
the wafers, wherein the front face was specular, free from
scratches, stains and adhering particles.
The backside of the remaining wafers did not show any loose dirt or
staining under ambient conditions, and the wafers were found
acceptable for device fabrication.
EXAMPLE 3
In cases of wax mounted polishing processes, the cleaning sequence
is modified in accordance with the following schedule.
A. expose wax mounted silicon wafer to aq. 5% NaClO for 5
minutes.
B. rinse for 5 minutes in flowing DI water, and dry.
C. demount and de-wax wafer with organic solvents such as trichloro
ethylene, and methyl or propylalcohol.
d. Immerse wafers for 5 minutes in aq. 5% NaClO solution.
E. rinse for 2 minutes in flowing DI water.
F. immerse wafers for 30 seconds in aq. NH.sub.4 OH.
G. rinse wafers in flowing DI water and dry.
Process steps E to G can be readily carried out in the commercially
available automated Corotek (Model No. COR II-CX) spin dryer.
However, its use requires careful control in the pressure of its
N.sub.2 environment to prevent staining of the wafer.
It may be noted that care must be taken to insure complete rinsing
in step E to prevent the formation of chloramine and other
oxidation products of ammonia in the wash liquid which are
undesirable from safety and pollution point of view.
Typical surface impurity analysis of wafers cleaned, in accordance
with this invention, and employing Emission Spectographic
techniques, showed the following results.
______________________________________ Impurity Level Element
atoms/cm.sup.2 ______________________________________ Al N.D. (not
detected) Na N.D. Ca N.D. Fe N.D. Cr N.D. Cu.sup.++ N.D. - 4
.times. 10.sup.12 Ti N.D. Mg N.D.
______________________________________
EXAMPLE 4
The cleaning schedule of Example 2 was repeated for six polished
silicon wafers with the exception that chromic acid was used in
Step A in place of sodium hypochlorite.
As in Example 2, these wafers also met the high specifications set
for them, as discussed in Example 2.
While this invention has been particularly described with reference
to the preferred embodiments thereof, it will be understood by
those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirt and scope of the invention.
* * * * *